A 90 isn't big enough for more than 1 front when they grow out, you really need a bigger tank and a group to keep them properly.
There are a few species of featherfins that will work in a 4' tank, and some of the smaller Cyp's or paracyp's as well, but the jumbo Cyp species need at least a 6' tank.
Tropheus are popular, and there's quite a variety of colors available, but they do best in a species tank.
Sexifasciatus are aggressive buggers, you wouldn't be able to keep much (if anything) more than a pair of those in a 90, though a single wouldn't be as likely to kill everything else.
Synodontis petricola or lucipinnis catfish are great in a group of 5 or more.